The photophysical properties of three octupolar chromophores containing planar triazatruxene (TAT) as the central electron donor with different electron-withdrawing groups in the tribranched arrangement have been systematically investigated by means of steady state and transient spectroscopy. The multidimensional intramolecular charge transfer (ICT) properties of these tribranched chromophores related to the observed two-photon absorption (TPA) properties are explored by estimating the TPA essential factors (Mge and Δμge). Besides the large Stokes shift between steady state absorption and fluorescence spectra in different polar solvents, photoinduced ICT was further demonstrated by quantum-chemical calculations and transient absorption measurements. Both quantum calculations and spectral experiments show that a multidimensional ICT occurs from the electron-rich core to the electron-deficient periphery of these TAT derivatives. The results of solvation effects and the dynamics of the excited states show that the excited states of these three chromophores tend to exhibit an excitation localization on one of the dipolar branches, which is beneficial to achieve large Mge and Δμge, thus leading to enhanced TPA properties.

Excited state solvation plays a very important role in modulating the emission behavior of fluorophores upon excitation. Here, the solvation effects on the local micro-environment around a fluorophore are proposed by investigating the fantastic emission behavior of a novel amyloid fibril marker, NIAD-4, in different alcoholic and aprotic solvents. In alcoholic solvents, high solvent viscosity causes an obvious enhancement of fluorescence because of the restriction of torsion of NIAD-4, where the formation of a non-fluorescent twist intramolecular charge transfer (TICT) state is suppressed. In aprotic solvents, high solvent polarity leads to a remarkable redshift of the emission spectra suggesting strong solvation. Surprisingly, an abnormal fluorescence enhancement of NIAD-4 is observed with increasing solvent polarity of the aprotic solvents, whereas solvent viscosity plays little role in influencing the fluorescence intensity. We conclude that such an abnormal phenomenon is originated from a solvation induced micro-viscosity enhancement around the fluorophore upon excitation which restricts the torsion of NIAD-4. Femtosecond transient absorption results further prove such a micro-viscosity increasing mechanism. We believe that this solvation induced micro-viscosity enhancement effect on fluorescence could widely exist for most donor–π–acceptor (D–π–A) compounds in polar solvents, which should be carefully taken into consideration when probing the micro-viscosity in polar environments, especially in complex bioenvironments.

The magnitude of intramolecular charge-transfer (ICT) in push–pull chromophores and the fraction of delocalized excitation in multibranched chromophores and conjugated polymers play a crucial role in high photovoltaic efficiency of a solar cell. In this work, we present a joint theoretical and experimental study aimed to understand the influence of thiophene moiety on photophysical properties of push–pull chromophores for solar cell application. It is found that insertion of a thiophene moiety as the conjugated bridge enhances the magnitude of ICT in push–pull chromophores due to the inductive effect of the thiophene moiety. In addition, introduction of a thiophene moiety as the conjugated side chain significantly increases transition dipole moment of the chromophore, and as a consequence, interchromophoric coupling is enhanced, giving rise to a larger fraction of delocalized excitation within multibranched chromophores. The results presented here show that introduction of a thiophene moiety in push–pull chromophores contributes to the improvement of the photophysical properties necessary for highly efficient solar cell performance.

Benzo[1,3]oxazine, an organic optical switching compound, is known to be in an equilibrium between its closed form (OX) and its open zwitterionic form (IN). Here we report a light-induced ring-closing mechanism of a hydrogen-bonded adduct (p-IN, partially protonated open isomer of benzo[1,3]oxazine) based on the observations of femtosecond and nanosecond transient absorption spectra in protic solvents. Femtosecond transient measurements upon visible excitation reveal the appearance of two states having different dynamical signatures. One corresponds to a conventional intramolecular charge transfer excited state. The other one is a concerted electron–proton transfer product (d-IN, embedded solvent molecule released from p-IN). Without steric hindrance, the main molecular structure tends to be planar upon excitation, and two intermediates, IN and OX, are involved in the sequential thermal transformation before the return to the ground state of p-IN. Specifically, in alcoholic solvents, d-IN converts to the original p-IN compound within 1 ms via the dominant pathway d-IN → IN → OX → p-IN and the side pathway d-IN → IN → p-IN, which is found to be feasible in energy; in contrast, in aqueous solution with increasing strength of intermolecular hydrogen bonding, the rate of the thermal transformation is enhanced by 1 order of magnitude.

Electron transfer (ET) is widely used for driving the processes that underlie the chemistry of life. However, our abilities to probe electron transfer mechanisms in proteins and design redox enzymes are limited, due to the lack of methods to site-specifically insert electron acceptors into proteins in vivo. Here we describe the synthesis and genetic incorporation of 4-fluoro-3-nitrophenylalanine (FNO2Phe), which has similar reduction potentials to NAD(P)H and ferredoxin, the most important biological reductants. Through the genetic incorporation of FNO2Phe into green fluorescent protein (GFP) and femtosecond transient absorption measurement, we show that photoinduced electron transfer (PET) from the GFP chromophore to FNO2Phe occurs very fast (within 11 ps), which is comparable to that of the first electron transfer step in photosystem I, from P700* to A0. This genetically encoded, low-reduction potential unnatural amino acid (UAA) can significantly improve our ability to investigate electron transfer mechanisms in complex reductases and facilitate the design of miniature proteins that mimic their functions.

ApcE(1–240) dimers with one intrinsic phycocyanobilin (PCB) chromophore in each monomer that is truncated from the core-membrane linker (ApcE) of phycobilisomes (PBS) in Nostoc sp. PCC 7120 show a sharp and significantly red-shifted absorption. Two explanations either conformation-dependent Förster resonance energy transfer (FRET) or the strong exciton coupling limit have been proposed for red-shifted absorption. This is a classic example of the special pair in the photosynthetic light harvesting proteins, but the mechanism of this interaction is still a matter of intense debate. We report the studies using single-molecule and transient absorption spectra on the interaction in the special pair of ApcE dimers. Our results demonstrate the presence of conformation-dependent FRET between the two PCB chromophores in ApcE dimers. The broad distributions of fluorescence intensities, lifetimes and polarization difference from single-molecule measurements reveal the heterogeneity of local protein–pigment environments in ApcE dimers, where the same molecular structures but different protein environments are the main reason for the two PCB chromophores with different spectral properties. The excitation energy transfer rate between the donor and the acceptor about (110 ps)−1 is determined from transient absorption measurements. The red-shifted absorption in ApcE dimers could result from more extending conformation, which shows another type of absorption redshift that does not depend on strong exciton coupling. The results here stress the importance of conformation-controlled spectral properties of the chemically identical chromophores, which could be a general feature to control energy/electron transfer, widely existing in the light harvesting complexes.

We report a comprehensive study on a newly synthesized perylenetetracarboxylic diimide (PDI) hexamer together with its corresponding monomer and dimer by means of steady-state absorption and fluorescence as well as femtosecond broadband transient absorption measurements. The structure of the PDI hexamer is nearly arranged in a 3-fold symmetry by three identical and separated dimers. This unique structure makes the excited state energy relaxation processes more complex due to the existence of two different intramolecular interactions: a strong interaction between face-to-face PDIs in dimers and a relatively weak interaction between the three separated PDI dimers. The steady-state spectra and the ground state structural optimization show that the steric effect plays a dominant role in keeping the formation of the face-to-face stacked PDI-dimer within the PDI-hexamer, indicating that some level of a pre-associated excimer had formed already in the ground state for the dimer in the hexamer. Femtosecond transient absorption experiments on the PDI hexamer reveal a fast (∼200 fs) localization process and a sequential relaxation to a pre-associated excimer trap state from the delocalized exciton state with about 1.2 ps after the initially delocalized excitation. Meanwhile, excitation energy transfer among the three separated dimers within the PDI-hexamer is also revealed by the anisotropic femtosecond pump–probe transient experiments, where the hopping time is about 2.8 ps. A relaxed excimer state is further formed in 7.9 ps after energy hopping and conformational relaxation.

The luminescent ligand protected metal clusters have attracted considerable attentions while the origin of the emission still remains elusive. As recently reported in our previous work, the rod-shaped Au25 cluster possesses a low photoluminescence quantum yield (QY = 0.1%), whereas substituting silver atoms for central gold atom in the rod-shaped Au25 cluster can drastically enhance the photoluminescence with high quantum yield (QY = 40.1%). To explore the enhancement mechanism of fluorescence, femtosecond transient absorption spectroscopy is performed to determine the electronic structure and ultrafast relaxation dynamics of the highly luminescent silver-doped AgxAu25–x cluster by comparing the excited state dynamics of doped and undoped Au25 rod cluster, it is found that the excited state relaxation in AgxAu25–x is proceeded with an ultrafast (∼0.58 ps) internal conversion and a subsequent nuclear relaxation (∼20.7 ps), followed by slow (7.4 μs) decay back to the ground state. Meanwhile, the observed nuclear relaxation is much faster in AgxAu25–x (∼20.7 ps) compared to that in undoped Au25 rod (∼52 ps). We conclude that it is the central Ag atom that stabilizes the charges on LUMO orbital and enhances the rigidity of AgxAu25–x cluster that leads to strong fluorescence. Meanwhile, coherent oscillations around ∼0.8 THz were observed in both clusters, indicating the symmetry preservation from Au cluster to Ag alloying Au clusters. The present results provide new insights for the structure-related excited state behaviors of luminescent ligand protected Ag alloying Au clusters.

Polynuclear Au(I) complexes continues to attract considerable attention because of their bright emissions in the visible wavelength, which hold promise in applications in luminescence, fluorescence sensing, and bioimaging. Despite various spectroscopic investigations on their steady state properties, detailed understanding of the origin of their emissions and excited state relaxations is still lacking. Here, we report femtosecond time-resolved transient absorption experiments combined with quantum chemical calculations on a brightly emissive [Au6Ag2(C)(dppy)6](BF4)4 cluster in different solvents. Global analysis on the transient absorption spectra based on a sequential model gives three spectral components: (1) excited state absorption (ESA) of 1MLCTAu state (τ = 1–3 ps); (2) ESA of 3MLCTAu state (τ = 11–40 ps), and (3) ESA of 3MLCTAg state (long-lived). By variation of the solvent’s polarity and hydrogen bonding ability, the relative population of the triplet MLCT states and the emission properties can be modulated. Especially in methanol, an additional site specific O–H···π bond is formed between methanol molecules and aromatic rings of ligands, which enhances the ultrafast nonradiative decay from the hydrogen bond stabilized 3MLCTAu state and reduces the population of the emissive 3MLCTAg state. The results presented here about the excited state dynamics of luminescent gold(I)–silver(I) cluster allow a deeper insight into the origin of their emissions by monitoring the population of the emissive 3MLCTAg state and dark 3MLCTAu state in different environments.

A new kind of tetrahydro[5]helicene-based imide dyes with intense fluorescence and large Stokes shifts in both solution and solid state were developed and theoretically investigated.

The exact interaction between Au cores and surface ligands remains largely unknown because of the complexity of the structure and chemistry of ligand/Au-core interfaces in ligand-protected Au nanoclusters (AuNCs), which are commonly found in many organic–inorganic complexes. Here, femtosecond transient absorption measurement of the excited-state dynamics of a newly synthesized phosphine-protected cluster [Au20(PPhpy2)10Cl4]Cl2 (1) is reported. Intramolecular charge transfer (ICT) from the Au core to the peripheral ligands was identified. Furthermore, we found that solvation strongly affected ICT at ligand/Au-core interfaces while by choosing several typical alcoholic solvents with different intrinsic solvation times, we successfully observed that excited-state relaxation dynamics together with displacive excited coherent oscillation of Au20 clusters were significantly modulated through the competition between solvation and surface trapping. The results provide a fundamental understanding of the structure–property relationships of the solvation-dependent core–shell interaction of AuNCs for the potential applications in catalysis, sensing and nanoelectronics.

The rotational dynamics of coumarin 153 (C153) have been investigated in a series of 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids with different alkyl chain lengths (alkyl = butyl, pentyl, hexyl, heptyl, octyl) ([Cnmim][PF6], n = 4–8) to examine the alkyl chain length dependent local viscosity of the microenvironment surrounding the probe molecules. The excimer-to-monomer fluorescence emission intensity ratio (IE/IM) of a well-known microviscosity probe, 1,3-bis(1-pyrenyl)propane (BPP), is also employed to study the microviscosity of [Cnmim][PF6] as a complementary measurement. The rotational dynamics of C153 show that at a certain length of the alkyl chain there are incompact and compact domains within [Cnmim][PF6], resulting in fast and slow components of C153 rotational dynamics. The microviscosities in different structural domains of [Cnmim][PF6] with different alkyl chain lengths are investigated by studying the fluorescence anisotropy decay of probe molecules. The obtained average rotation time constants show that with an increase in the length of the alkyl chain, the microviscosity of [Cnmim][PF6] is obviously increased first and then slightly decreased. The steady state fluorescence measurements with the microviscosity probe of BPP further prove that the microviscosity is not increased as much as expected when ionic liquids [Cnmim][PF6] have a relatively long alkyl chain. The different heterogeneous structures of [Cnmim][PF6] with different lengths of the alkyl chain are proposed to interpret the unusual microviscosity behaviors.

Determination of the excited-state dynamics of carotenoids has attracted considerable interest, engendering a number of controversial hypotheses because of the strongly overlapping spectral peaks and complicated dynamics of transient species. In the present work, aiming for better understanding the complexity of excited-state processes in carotenoids, excited-state dynamics of all-trans-β-carotene in ethanol was investigated by femtosecond pump–probe spectroscopy. Following the excitation of the strongly allowed S2 state of the β-carotene, transient absorption spectra were recorded in the visible spectral range. For comparison, the time-resolved transient absorption spectra are analyzed in a conventional way, fitting kinetic traces with a multi-exponential function at chosen wavelengths from obtained spectra, and then again by means of the soft-modeling multivariate curve resolution alternating least-squares analysis (MCR-ALS) method for modeling pure profiles and the generalized two-dimensional (2D) correlation spectroscopy data analysis for providing additional information on the dynamics of spectral features. MCR-ALS analysis shows that both the dynamics of the S* state, identified using the 2D correlation spectra, and the S1 state develop on a different timescale than the relaxation of the vibrationally hot S1v′ state. Hot S1v′ and S* states are shown to have different species associated difference spectra. Results of our analysis indicate that the S* state observed in this work is not the hot S1v′ state but instead a separate singlet state.

It is accepted that the monolayer ligand shell in monolayer-protected gold nanoclusters (MPCs) plays an important role in stabilizing the metal core structure. Previous reports have shown that the core and shell do not interact chemically, and very few studies investigating the intramolecular charge transfer (ICT) between the core and ligand shell in clusters have been reported. The underlying excited state relaxation mechanisms about the influence of solvents, the optically excited vibration, and the roles of the core and shell in charge transfer remain unknown to a large extent. Here we report a femtosecond transient absorption study of a Au20(SR)16 (R = CH2CH2Ph) cluster in toluene and tetrahydrofuran. The ICT from the outside shell to the inside core upon excitation in Au20(SR)16 is identified. The observed solvation-dependent oscillations in different solvents further confirm the photoinduced ICT formation in Au20(SR)16. The results provide a fundamental understanding of the structure–property relationships about the solvation-dependent core–shell interaction in Au MPCs.

Hofmeister ion-specific effects on optical properties of a water-soluble cationic polymer, poly(3-alkoxy-4-methylthiophene) (PMNT), are investigated by means of absorption, resonance Raman spectroscopy, and molecular dynamic simulations. It is found that the ionochromism of conjugated polyelectrolytes PMNT obeys Hofmeister series with high optical sensitivity, while the spectral changes result from the different electrostatic interactions and the conformational change of the cationic PMNT in different salt solutions. As a result, UV–vis absorption spectra exhibit almost no shift of absorption of PMNT in the presence of SO42–, F–, etc., whereas a red-shifted absorption of PMNT with I–, SCN– is clearly observed. To gain a deeper understanding of the nature of these anion-dependent chromic phenomena, ab initio calculations and molecular dynamics (MD) simulations are carried out to present the microscopic insights, that the Hofmeister effect occurs at the PMNT/water interface through the direct (hydrophobic, hydrophilic, and electrostatic) interactions between the anions and PMNT backbone. It is found that the salting-in anions like I– strongly suppress the hydrophobic collapse of PMNT backbone, leading to more extended and ordered PMNT backbone with red-shifted absorption, and the salting-out anions like F– strongly avoid the hydrophobic PMNT backbone, keeping a random-coiled configuration of PMNT backbone without obvious absorption changes in KF solution. The existence of ordered and disordered backbone configurations is further verified by monitoring the main in-plane skeleton Raman modes (C═C and C–C stretch) of PMNT in various salt solutions. The results presented here could provide a fundamental understanding of salt effects on chemical and biological processes occurring at the macromolecular/water interface, and then may potentially stimulate many chemical and biological applications.

In order to better understand the nature of intramolecular charge and energy transfer in multibranched molecules, we have synthesized and studied the photophysical properties of a monomer quadrupolar chromophore with donor–acceptor–donor (D–A–D) electronic push–pull structure, together with its V-shaped dimer and star-shaped trimers. The comparison of steady-state absorption spectra and fluorescence excitation anisotropy spectra of these chromophores show evidence of weak interaction (such as charge and energy transfer) among the branches. Moreover, similar fluorescence and solvation behavior of monomer and branched chromophores (dimer and trimer) implies that the interaction among the branches is not strong enough to make a significant distinction between these molecules, due to the weak interaction and intrinsic structural disorder in branched molecules. Furthermore, the interaction between the branches can be enhanced by inserting π bridge spacers (−C═C– or −C≡C−) between the core donor and the acceptor. This improvement leads to a remarkable enhancement of two-photon cross-sections, indicating that the interbranch interaction results in the amplification of transition dipole moments between ground states and excited states. The interpretations of the observed photophysical properties are further supported by theoretical investigation, which reveal that the changes of the transition dipole moments of the branched quadrupolar chromophores play a critical role in observed the two-photon absorption (2PA) cross-section for an intramolecular charge transfer (ICT) state interaction in the multibranched quadrupolar chromophores.

The rotational dynamics of coumarin 153 (C153) in imidazolium-based ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and tetraethylene glycol dimethyl ether (TEGDME) mixtures across all mole fractions have been investigated to determine the local viscosity of the microenvironment surrounding the probe molecules. The excimer-to-monomer fluorescence emission intensity ratio (IE/IM) of a well-known microviscosity probe, 1,3-bis(1-pyrenyl)propane (BPP), is also employed to study the microviscosity of the mixtures as a complementary measurement. The rotational dynamics of C153 show that there are incompact and compact domains within the heterogeneous structural [bmim][PF6], resulting in fast and slow components of C153 rotational dynamics. The microviscosity in different structural domains of [bmim][PF6] before and after adding cosolvent TEGDME with different mole fractions is further investigated by studying the fluorescence anisotropy decay of probe molecules. The obtained average rotation time constants show that the microviscosity of [bmim][PF6] is enhanced after mixing with a certain amount of TEGDME, although the bulk viscosity of TEGDME itself is much lower than that of the ionic liquid. This unusual behavior of microviscosity enhancement is further proven by the steady state fluorescence measurement with the microviscosity probe of BPP. The microviscosity enhancement is reasonably demonstrated by the fast time constant of C153 rotational dynamics and the departure between the experimentally observed and calculated ratio of IE/IM of BPP, which shows that this effect is most pronounced at intermediate mole fractions of the [bmim][PF6] and TEGDME mixtures. The strengthening effects caused by the molecular interactions between TEGDME and structural heterogeneous ionic liquid [bmim][PF6] are proposed to interpret the unusual microviscosity behaviors.

The absorption spectra and intramolecular charge transfer (CT) properties of terminal donor/acceptor-substituted all-trans-α,ω-diphenylpolyenes (DPE) and α,ω-diphenylpolyynes (DPY) molecules with different conjugated bridge length and substitution modes were investigated by using quantum chemical calculations. We calculated the ground state structures and energy of two series of terminal donor/acceptor DPE and DPY by DFT method. The dependence of conjugation length and substitution modes of the electronic absorption spectra was obtained by TDDFT calculation. The hybrid-GGA XC-functional PBE0 employed in this work was selected from several functionals by comparing the calculated electronic spectral data with experimental value. The CIS-based generalized Mulliken-Hush (GMH) approach was further used to calculate coupling values HAD of the CT process. The calculation shows that both the HOMO–LUMO energy gaps and average bond length alternations between unsaturated multiple (CC and CC) and saturated single bonds (C–C) decrease regularly with the extension of conjugation. The effective conjugated length (ECL) of DPE and DPY with the same order MM > MP/PM > PP is found together with the regular red shift of the electronic absorption spectra with the extension of conjugation, resulting from the different π-electron delocalization and conjugation efficiency. The GMH analysis further suggests that the CT process in both DPE and DPY is predominated by the through-bond mechanism. The remarkable difference of the conjugated length dependence of squared CT coupling between substituted DPE and DPY is the result of the energetic matching degree of the frontier molecular orbitals between donor/acceptor and the conjugated bridge.

The fluorescence depletion dynamics of Rhodamine 700 (R-700) molecules in room temperature ionic liquids (RTILs) 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) and 1-hydroxyethyl-3-methylimidazolium tetrafluoroborate ([HOemim][BF4]) were investigated to determine the local viscosity of the microenvironment surrounding the fluorescent molecules, which is induced by strong hydrogen bonding interaction between cationic and anionic components in RTILs. The solvation and rotation dynamics of R-700 molecules in RTILs show slower time constants relative to that in conventional protic solvents with the same bulk viscosity, indicating that the probe molecule is facing a more viscous microenvironment in RTILs than in conventional solvents because of the strong hydrogen bonding interaction between cationic and anionic components. In addition, this effect is more pronounced in hydroxyl-functionalized ionic liquid than in the regular RTIL due to the presence of a hydroxyl group as a strong hydrogen bonding donor. The hydrogen-bonding-induced local viscosity enhancement effect related to the heterogeneity character of RTILs is confirmed by the nonexponential rotational relaxation of R-700 determined by time-correlated single photon counting (TCSPC). The geometry of hydrogen bonding complexes with different components and sizes are further optimized by density functional theory methods to show the possible hydrogen-bond networks. A model of the hydrogen-bonding network in RTILs is further proposed to interpret the observed specific solvation and local viscosity enhancement effect in RTILs, where most of the fluoroprobes exist as the free nonbonding species in the RTIL solutions and are surrounded by the hydrogen-bonding network formed by the strong hydrogen-bonding between the cationic and anionic components in RTIL. The optimized geometry of hydrogen bonding complexes with different components and sizes by density functional theory methods confirms the local viscosity enhancement effect deduced from fluorescence depletion and TCSPC experiments. The calculated interaction energies reveal the existence of the stronger hydrogen bonding network in RTILs (especially in hydroxyl-functionalized ionic liquid) than that in conventional protic solvent, which leads to the enhancement effect of local microviscosity, and therefore leads to the slow solvation and rotation dynamics of probe molecules observed in RTILs.

We introduce a new rhodamine-based fluorescent chemosensor, FD8 which exhibits a distinct two-photon excited fluorescence (TPEF) on/off characteristic upon binding Cr3+ ions. By coordination with metal cation, conformation of FD8 changes from spirocyclic to open-ring, resulting in remarkable enhancement of absorption and fluorescence both in one- and two-photon excitations. As a result, a 29-fold enhancement of two-photon excited fluorescent intensity was observed when 10 eq. Cr3+ was added to the FD8 solution. The detection limit of Cr3+ cation concentration down to 1 μM (0.01 eq. of FD8) was achieved under our experimental condition. Besides the excitation within ultraviolet regime by fluorescence resonance energy transfer (FRET) mechanism, the TPEF on/off behavior further extends the excitation to near infrared regime (the biological optimal window of 700–1200 nm), and shows more effective sensitivity. The broad excitation wavelength, on/off fluorescence and high selectivity to Cr3+ enable FD8 to be a powerful Cr3+ cation sensor with potential application, especially in biological detection. To the best of our knowledge, this is the first report about two-photon fluorescent sensor for Cr3+ ions.

Photoinduced electron transfer (ET) between C60 and porphyrin (P) in a new polymer containing porphyrin, poly(p-phenylenevinylene), and pendant fullerene units has been investigated by nanosecond transient absorption and phosphorescence spectroscopy. Compared to the physically doping material systems, binding porphyrin/C60 through chemical bonds in a polymer detains the formation of the triplet states of porphyrins and C60. The formation of intermediate charge transfer state (CSS) of P+-C60− was observed, which led to the delayed formation of triplet states of porphyrins and C60. The reduced opto-electronic properties, such as optical limiting performance, were also observed, which resulted from the delayed formation of triplet states. The results presented in this article are significant in understanding the complicated spectral characteristics of the triplet state and charge transfer of the porphyrin and C60 complexes, and are therefore related to the controllable performance of the new materials in applications.

We present the structure-dependent nonlinear optical (NLO) properties of fully conjugated tri(perylene bisimides) (triPBIs) toward the understanding of the role of conformational flexibility and π-electron conjugation in molecular NLO properties of model graphene-nanoribbon (GNR)-like molecules. In the present paper, we report the NLO absorption properties of the triPBIs in toluene excited at 532 nm with nanosecond laser pulses, where the observed transient excited state is determined to be a triplet and presented in the nonlinear process similar to the NLO properties that occur in C60. As a result, the all-optical switching in both visible and near-infrared regions upon excitation at 532 nm was demonstrated, suggesting that the chemically synthesized model GNRs act well as smart all-optical switching devices without the need of external control. Furthermore, Raman spectral measurement was further used to characterize the conjugated structure properties of model compounds of functionalized graphene nanoribbons (F-GNRs), while the dispersion and splitting of the G-band and D-band in both frequency and intensity can help to distinguish the π-conjugation and conformational flexibility of the two different triPBI isomers, showing the opportunity to tailor their optoelectronic properties by precisely controlling the edge orientation, edge width, and chemical termination of the edges in the synthesized F-GNRs.

The nature of optical excitation and the degree of intramolecular charge transfer (ICT) as well as the dynamics of excited ICT states of two new tribranched donor−π−acceptor molecules with acceptor-terminated (DA3) and acceptor-centered (AD3) geometries have been investigated by steady-state and femtosecond time-resolved stimulated emission fluorescence depletion (FS TR-SEP FD) measurements in different polar solvents. The interpretation of the experimental results is based on the comparative investigation of the two D−π−A compounds with respect to the model monomer counterpart (DA). The larger solvatochromic effects and stronger solvent dependence of spectral properties of DA3 than that of AD3 indicate that the excited ICT state of DA3 possesses higher polarity and larger dipole moments compared to those of AD3. The similarity of absorption and strong solvent-dependent fluorescence spectra of DA3 and DA reveals that the excited-state properties of DA3 are identical to that of the model DA, which localized on one of the branches in DA3. In contrast to DA3, the large red shift in the absorption and the small Stokes shift of AD3 suggest the formation of a delocalized ICT state to a certain extent in the excited state of AD3. The dynamic behavior of excited ICT states for all three compounds are also investigated by femtosecond time-resolved stimulated emission depletion (FS TR-SEP FD) measurements, where the excited-state relaxations are highly dependent on both solvent polarity and the polar degree of the excited ICT states. Furthermore, the steady-state fluorescence excitation anisotropy shows that the intramolecular excitation transfer among the three disorder-induced localized ICT states with nondegenerate transition dipole moments is involved within DA3. Compared to DA3, a substantial red shift in the absorption of AD3 results from the formation of a delocalized ICT state, where the specific excitation anisotropy spectrum shows that the excitation energy is mainly redistributed between the localized ICT state and the delocalized ICT state.

We report a series of stiff dendrimers (referred to as T1, T2, T3, and T4) that have both gigantic two-photon absorption (TPA) cross sections up to 25 000 GM and strong two-photon excited fluorescence (TPEF) with fluorescence quantum yield of ∼0.5. The large TPA cross sections and high quantum yields of these dendrimers are directly related to their geometrical structures, where the polycyclic aromatic pyrene is chosen as the chromophoric core because of its planar and highly π-conjugated structure, fluorene moieties as dendrons extend the conjugation length through the planar structure, and carbazole moieties are modified at three-, six-, and nine-positions as electron donor. All of these groups are linked with acetylene linkage for effective π-electron delocalization, leading to large TPA cross section and high fluorescence quantum yield. The spectral properties of all dendrimers are investigated by one- and two-photon excitations. Furthermore, steady-state fluorescence excitation anisotropy and quantum chemical calculation are also employed to determine the structure-related mechanism of these dendrimers with gigantic TPA cross sections and high TPEF efficiency. We then show that the improvement of branched chains in the T-series dendrimers enhances the light-harvesting ability. The core emission spectra, fluorescence quantum yield, and fluorescence lifetime are almost invariable by directly exciting the dendrons. These results will provide a guideline for the design of useful two-photon materials with structural motifs that can enhance the TPA cross-section and fluorescence quantum yield of a molecule without causing a red shift of the one- and two-photon excitation wavelengths for specific applications.

Single-molecule (SM) spectroscopy has been conducted to study exciton-like intramolecular charge transfer (ICT) coupling dynamics in two model dendritic systems containing branched ICT interactions. The strong coupling and stepwise photobleaching of the ICT branches in the dendrimers, which depend on the torsional disorder, have been demonstrated at the SM level. The fluorescence from the delocalized ICT excited state over two branches in push−pull molecules, which cannot be distinguished by means of conventional experiments probing the average behavior of large ensembles of molecules, has been observed at the SM level.

We report a newly synthesized polymer of a star-shaped porphyrin compound (TPA-FxP) with four oligofluorene arms at its meso positions with the pronounced enhancement of the two-photon properties and the generation of singlet oxygen by utilizing the two-photon excited fluorescence resonance energy transfer. The steady-state spectra and transient triplet-triplet absorption spectra give evidence that the enhanced two-photon absorption cross section results from not only the through-space energy transfer (Förster) but also the through-bond energy transfer between conjugated peripheral oligofluorene arms and the porphyrin core. The two-photon absorption cross section at 780 nm up to 3360 GM (1 GM = 10−50 cm4·s/photon) of TPA-FxP was obtained, which is comparable to the highest values reported from other similar chemically modified porphyrin core compounds. Furthermore, the enhanced production of singlet oxygen under two-photon absorption conditions is also reported.

The intersystem crossing and isomerization dynamics of free-Cy3, Cy3-ssDNA, free-Cy5 and Cy5-ssDNA are obtained through simple analysis of rapid on/off blinking from single molecule fluorescence intensity time-traces and the fluorescence correlation spectroscopy (FCS). The on- and off-times observed in fluorescence time traces of single cyanine dyes are due to the formation of the triplet state and isomerization, where both the interaction with DNA and long central polymethine chain of cyanine dyes increase the barriers of isomerization, leading to long off-time. The results indicate that the single molecule fluorescence fluctuation together with the resulting second autocorrelation analysis are powerful methods for determining the triplet state and isomerization dynamics, which could be the simple techniques and complementary to other spectroscopic techniques, such as fluorescence decay measurement and laser flash photolysis to study the photophysical processes of complex molecules.

The spectroscopic properties of hydrogen-bond modulated cofacial porphyrin dimers in different polar solvents are reported. UV–Vis absorption, fluorescence and fluorescence lifetime measurements provide information regarding the exciton interaction of the cofacial porphyrin dimers in different polar solvents. The results indicate that the absorptions of the Soret band of the cofacial dimer can be slightly turned by changing the polarities of solvents. The strong intramolecular hydrogen bonds play an important role to enforce the adoption of the H-aggregation mode, and then finely turn the spectral properties with different polar solvents.

The fluorescence properties of three novel compounds are investigated by one- and two-photon excitations. These novel compounds are formed by self-assembly of pyrrol-2-yl-methyleneamines with zinc (II), which show perfectly symmetries in molecular structures. Intensity dependent fluorescence properties of both one- and two-photon-induced fluorescence excited by an 800 nm, 70 fs laser beam are reported. The fluorescence spectra and the fluorescence lifetimes of these new compounds obtained by two-photon excitation are similar to those obtained by one-photon excitation, which indicate that the two-photon-induced photodynamic processes of these complexes are similar to one-photon-induced photodynamic processes. The two-photon excitation cross-sections of these new compounds are also measured at 800 nm. The large two-photon excitation cross-sections of these three novel compounds indicate their potential properties for two-photon applications in nonlinear optics and biomolecular imaging fields.

A new kind of tetrahydro[5]helicene-based imide dyes with intense fluorescence and large Stokes shifts in both solution and solid state were developed and theoretically investigated.

The photophysical properties of three octupolar chromophores containing planar triazatruxene (TAT) as the central electron donor with different electron-withdrawing groups in the tribranched arrangement have been systematically investigated by means of steady state and transient spectroscopy. The multidimensional intramolecular charge transfer (ICT) properties of these tribranched chromophores related to the observed two-photon absorption (TPA) properties are explored by estimating the TPA essential factors (Mge and Δμge). Besides the large Stokes shift between steady state absorption and fluorescence spectra in different polar solvents, photoinduced ICT was further demonstrated by quantum-chemical calculations and transient absorption measurements. Both quantum calculations and spectral experiments show that a multidimensional ICT occurs from the electron-rich core to the electron-deficient periphery of these TAT derivatives. The results of solvation effects and the dynamics of the excited states show that the excited states of these three chromophores tend to exhibit an excitation localization on one of the dipolar branches, which is beneficial to achieve large Mge and Δμge, thus leading to enhanced TPA properties.

Excited state solvation plays a very important role in modulating the emission behavior of fluorophores upon excitation. Here, the solvation effects on the local micro-environment around a fluorophore are proposed by investigating the fantastic emission behavior of a novel amyloid fibril marker, NIAD-4, in different alcoholic and aprotic solvents. In alcoholic solvents, high solvent viscosity causes an obvious enhancement of fluorescence because of the restriction of torsion of NIAD-4, where the formation of a non-fluorescent twist intramolecular charge transfer (TICT) state is suppressed. In aprotic solvents, high solvent polarity leads to a remarkable redshift of the emission spectra suggesting strong solvation. Surprisingly, an abnormal fluorescence enhancement of NIAD-4 is observed with increasing solvent polarity of the aprotic solvents, whereas solvent viscosity plays little role in influencing the fluorescence intensity. We conclude that such an abnormal phenomenon is originated from a solvation induced micro-viscosity enhancement around the fluorophore upon excitation which restricts the torsion of NIAD-4. Femtosecond transient absorption results further prove such a micro-viscosity increasing mechanism. We believe that this solvation induced micro-viscosity enhancement effect on fluorescence could widely exist for most donor–π–acceptor (D–π–A) compounds in polar solvents, which should be carefully taken into consideration when probing the micro-viscosity in polar environments, especially in complex bioenvironments.

We report a comprehensive study on a newly synthesized perylenetetracarboxylic diimide (PDI) hexamer together with its corresponding monomer and dimer by means of steady-state absorption and fluorescence as well as femtosecond broadband transient absorption measurements. The structure of the PDI hexamer is nearly arranged in a 3-fold symmetry by three identical and separated dimers. This unique structure makes the excited state energy relaxation processes more complex due to the existence of two different intramolecular interactions: a strong interaction between face-to-face PDIs in dimers and a relatively weak interaction between the three separated PDI dimers. The steady-state spectra and the ground state structural optimization show that the steric effect plays a dominant role in keeping the formation of the face-to-face stacked PDI-dimer within the PDI-hexamer, indicating that some level of a pre-associated excimer had formed already in the ground state for the dimer in the hexamer. Femtosecond transient absorption experiments on the PDI hexamer reveal a fast (∼200 fs) localization process and a sequential relaxation to a pre-associated excimer trap state from the delocalized exciton state with about 1.2 ps after the initially delocalized excitation. Meanwhile, excitation energy transfer among the three separated dimers within the PDI-hexamer is also revealed by the anisotropic femtosecond pump–probe transient experiments, where the hopping time is about 2.8 ps. A relaxed excimer state is further formed in 7.9 ps after energy hopping and conformational relaxation.

The exact interaction between Au cores and surface ligands remains largely unknown because of the complexity of the structure and chemistry of ligand/Au-core interfaces in ligand-protected Au nanoclusters (AuNCs), which are commonly found in many organic–inorganic complexes. Here, femtosecond transient absorption measurement of the excited-state dynamics of a newly synthesized phosphine-protected cluster [Au20(PPhpy2)10Cl4]Cl2 (1) is reported. Intramolecular charge transfer (ICT) from the Au core to the peripheral ligands was identified. Furthermore, we found that solvation strongly affected ICT at ligand/Au-core interfaces while by choosing several typical alcoholic solvents with different intrinsic solvation times, we successfully observed that excited-state relaxation dynamics together with displacive excited coherent oscillation of Au20 clusters were significantly modulated through the competition between solvation and surface trapping. The results provide a fundamental understanding of the structure–property relationships of the solvation-dependent core–shell interaction of AuNCs for the potential applications in catalysis, sensing and nanoelectronics.

Determination of the excited-state dynamics of carotenoids has attracted considerable interest, engendering a number of controversial hypotheses because of the strongly overlapping spectral peaks and complicated dynamics of transient species. In the present work, aiming for better understanding the complexity of excited-state processes in carotenoids, excited-state dynamics of all-trans-β-carotene in ethanol was investigated by femtosecond pump–probe spectroscopy. Following the excitation of the strongly allowed S2 state of the β-carotene, transient absorption spectra were recorded in the visible spectral range. For comparison, the time-resolved transient absorption spectra are analyzed in a conventional way, fitting kinetic traces with a multi-exponential function at chosen wavelengths from obtained spectra, and then again by means of the soft-modeling multivariate curve resolution alternating least-squares analysis (MCR-ALS) method for modeling pure profiles and the generalized two-dimensional (2D) correlation spectroscopy data analysis for providing additional information on the dynamics of spectral features. MCR-ALS analysis shows that both the dynamics of the S* state, identified using the 2D correlation spectra, and the S1 state develop on a different timescale than the relaxation of the vibrationally hot S1v′ state. Hot S1v′ and S* states are shown to have different species associated difference spectra. Results of our analysis indicate that the S* state observed in this work is not the hot S1v′ state but instead a separate singlet state.

The absorption spectra and intramolecular charge transfer (CT) properties of terminal donor/acceptor-substituted all-trans-α,ω-diphenylpolyenes (DPE) and α,ω-diphenylpolyynes (DPY) molecules with different conjugated bridge length and substitution modes were investigated by using quantum chemical calculations. We calculated the ground state structures and energy of two series of terminal donor/acceptor DPE and DPY by DFT method. The dependence of conjugation length and substitution modes of the electronic absorption spectra was obtained by TDDFT calculation. The hybrid-GGA XC-functional PBE0 employed in this work was selected from several functionals by comparing the calculated electronic spectral data with experimental value. The CIS-based generalized Mulliken-Hush (GMH) approach was further used to calculate coupling values HAD of the CT process. The calculation shows that both the HOMO–LUMO energy gaps and average bond length alternations between unsaturated multiple (CC and CC) and saturated single bonds (C–C) decrease regularly with the extension of conjugation. The effective conjugated length (ECL) of DPE and DPY with the same order MM > MP/PM > PP is found together with the regular red shift of the electronic absorption spectra with the extension of conjugation, resulting from the different π-electron delocalization and conjugation efficiency. The GMH analysis further suggests that the CT process in both DPE and DPY is predominated by the through-bond mechanism. The remarkable difference of the conjugated length dependence of squared CT coupling between substituted DPE and DPY is the result of the energetic matching degree of the frontier molecular orbitals between donor/acceptor and the conjugated bridge.

The rotational dynamics of coumarin 153 (C153) have been investigated in a series of 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids with different alkyl chain lengths (alkyl = butyl, pentyl, hexyl, heptyl, octyl) ([Cnmim][PF6], n = 4–8) to examine the alkyl chain length dependent local viscosity of the microenvironment surrounding the probe molecules. The excimer-to-monomer fluorescence emission intensity ratio (IE/IM) of a well-known microviscosity probe, 1,3-bis(1-pyrenyl)propane (BPP), is also employed to study the microviscosity of [Cnmim][PF6] as a complementary measurement. The rotational dynamics of C153 show that at a certain length of the alkyl chain there are incompact and compact domains within [Cnmim][PF6], resulting in fast and slow components of C153 rotational dynamics. The microviscosities in different structural domains of [Cnmim][PF6] with different alkyl chain lengths are investigated by studying the fluorescence anisotropy decay of probe molecules. The obtained average rotation time constants show that with an increase in the length of the alkyl chain, the microviscosity of [Cnmim][PF6] is obviously increased first and then slightly decreased. The steady state fluorescence measurements with the microviscosity probe of BPP further prove that the microviscosity is not increased as much as expected when ionic liquids [Cnmim][PF6] have a relatively long alkyl chain. The different heterogeneous structures of [Cnmim][PF6] with different lengths of the alkyl chain are proposed to interpret the unusual microviscosity behaviors.

ApcE(1–240) dimers with one intrinsic phycocyanobilin (PCB) chromophore in each monomer that is truncated from the core-membrane linker (ApcE) of phycobilisomes (PBS) in Nostoc sp. PCC 7120 show a sharp and significantly red-shifted absorption. Two explanations either conformation-dependent Förster resonance energy transfer (FRET) or the strong exciton coupling limit have been proposed for red-shifted absorption. This is a classic example of the special pair in the photosynthetic light harvesting proteins, but the mechanism of this interaction is still a matter of intense debate. We report the studies using single-molecule and transient absorption spectra on the interaction in the special pair of ApcE dimers. Our results demonstrate the presence of conformation-dependent FRET between the two PCB chromophores in ApcE dimers. The broad distributions of fluorescence intensities, lifetimes and polarization difference from single-molecule measurements reveal the heterogeneity of local protein–pigment environments in ApcE dimers, where the same molecular structures but different protein environments are the main reason for the two PCB chromophores with different spectral properties. The excitation energy transfer rate between the donor and the acceptor about (110 ps)−1 is determined from transient absorption measurements. The red-shifted absorption in ApcE dimers could result from more extending conformation, which shows another type of absorption redshift that does not depend on strong exciton coupling. The results here stress the importance of conformation-controlled spectral properties of the chemically identical chromophores, which could be a general feature to control energy/electron transfer, widely existing in the light harvesting complexes.